Composite leaf spring

I've just started doing some research into composite leaf springs, and found this: http://www.enggjournals.com/ijet/docs/IJET14-06-01-104.pdf. I'm tempted to go for a Topolino type front suspension on my Morgan style cyclekart, as it's simpler to build than sliding pillar. Lack of easily available leaf springs was putting me off, but as I manufacture model boat hulls, making a fibreglass spring would be no problem.

Hmm. The author's unfamialarity with basic fabric nomenclature and composite engineering make this look like a high school or garage project. He appears to use woven (plain weave) cloth in the 0/90 direction, which he calls bi-directional. He uses no stitched (unwoven) cloth, when this application screams for some uni-directional cloth in the 0 direction. No sane composite engineer would drill a big hole in the center of the span without significantly adding some local reinforcement.

Not having built a spring before, but lots of other composite parts, I'd suggest the first attempt to get something more workable would be a lamination schedule that has a layer of woven cloth on the bias (+/-45 PW) on the outer faces, then inside those build 90% of the thickness in 0 degree uni fibers. If you plan to put holes anywhere, add a couple small patches of +/-45 cloth interspersed in the laminate, and consider widening the part slightly to compensate for the cut fibers. Carbon is not "black aluminum"- it is very directional, and this needs to used to advantage, rather than ignored.

His laminating jig is fine, but just clamping the laminate between two plastic lined surfaces (wood, aluminum, plywood) that are bent against some blocks is all you need. Make sure you can get even pressure and work out any air bubbles. Cut your part to shape with a diamond blade, or masonry-cutting disc in a grinder.

All this thinking and writing makes me want to head to the shop and give it a try- stay tuned!

Didn't have time to study it fully, agreed a slavish copy of a steel spring as a composite one is not good design. On the other hand that was what I found interesting, as it gives a direct size for size comparison of spring rates.

What I'd be inclined to do is make a plain blade, and either thicken the outer ends for bolted eyes, or add eyes post cure and trimming.

Thanks for taking the time to share your thoughts on a lamination schedule, my experience is purely non critical laminating in mat. I take it 0 degrees is along the axis of the spring, I'll probably try it with what I've got to hand, mat and glass tape initially, if needs be I can always refine it later.

As I'm thinking of a flat spring it'll be easy to make it up with flat boards, and vac bag the whole thing.

I talked to the guy making the commercial composite leaf springs for cars, mostly corvette, I asked for one for my spitfire. He said sure, he has one. I asked how he dealt with the swing axle, he didn't know and said he'll get back to me, I'm still waiting.

I would hesitate to be critical of the terminology used which is out of India. As I earned my living with fibreglass/glass reinforced plastics/kevlar/aramid fibre/graphite/carbon fibre et all, I am very aware that the very same product can have a multiplicity of names in various parts of the world. Even cloth weights prior to the more universal metric weights creates difficulties. I was responsible for building some Olympic Class Tornado catamarans that were consistently coming out too light. Then we realised that US clothe weights and the UK based ones were , while nominally the same, were different as the wight was ounces per 28" wide cloth and the other 36" wide.
No I have not read the article. Personally I have made things with "pressure moulded" as this was, but unless you can bleed off the excess resin I find it a not too satisfactory method of controlling resin/glass ratios. Vacuum with bleed cloth is a lot better, and resin infusion even more so.
Are composite springs an option, you bet. Room temp cure is marginal, temp control markedly better. (IMHO). If I am wrong, why do the resin manufacturers quote ALL their figures with a recommended temperature?

Not sure if it helps but I watched a professional crossbow maker set up to make a crossbow bow. He had a 2 1/2 inch deep plywood form he used with 3/4 ply base plate. The center of the form was cut out with the shape he wanted the bow to be on one side. The Opening was about 1 inch wide. 2 inches deep and Glued and Screwed to the very sturdy base.

He layered up his layers of cloth and resin on Wax paper which he folded ove when he had the right number of layers set up and placed in the form against the good side.
There was already a length of fire hose plugged at both ends placed in the form. When he had everything just right he would inflate the firehose and apply uniform pressure across the whole form. He used slow hardening resin and walked away for a day or two.

He was a very successful Crossbow maker. Similar idea may work for composite shocks. For the more powerful Crossbows he used steel or aluminium.

Thanks for the input Steve, remembered someone had tried it but couldn't remember who. I'll certainly try a test spring, got a choice of glass reinforcing here, tape, mat, and chopped strand, so I'll probably try those first. Slush moulding is easiest so I may start with that, just to see how it goes.

In reply to # 22093 by PDXBryan
This is an excellent thread and very interesting.
I've had an idea about repurposing old, out dated cross-country skis that I've had laying around for years. Would that be possible?

I may have missed it, but I didn't see any mention of carbon fiber.
For five years prior to my retiring from Fermilab, I was involved in
building many carbon fiber structures that went into the target areas
of the experiment halls. We used carbon fiber because of its incredable
strength which allowed us to keep the mass in the area of the detectors
at a minimum.
All of the material that I worked with was prepreg fabric. A layup might
have consisted of a half dozen layers of .005" material which we vacuum
bagged before and during its time in the curing oven.
Even though these structures were only 1/32" thick they would support
the weight of a human. An extremely light spring could be made from
this material.